My car has the factory AM-FM receiver and a CD unit below it. I am considering installing an XM satellite receiver or something. I do not need another CD unit. I am considering the XM Commander which should interconnect with the factory receiver, but I am uncertain of this. My question is this: Does the XM unit just sit on top of the dash and not replace anything? I am a newbie at this. Any informed help will be appreciated. Regards, Hoyt